This application is related to Japanese Patent Application No. 2001-377603 filed on Dec. 11, 2001, whose priority is claimed under 35 USC xc2xa7 119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a semiconductor device and its production process, more particularly, a semiconductor device using a semiconductor substrate into which strain is introduced by providing a SiGe film and a process of producing the semiconductor device.
2. Description of Related Art
For the purpose of improving the mobility of carriers (electrons or holes) passing through channel regions, there is known a technique of forming a strained SiGe pseudomorphic film on a Si substrate, relaxing the strain of the film caused by lattice mismatch between the film and the substrate by introducing misfit dislocation, and forming a Si film as a cap layer. The Si film is strained by the SiGe film having a larger lattice constant, thereby changing a band structure and improving the mobility of carriers.
For relaxing the strain of the SiGe film, there is known a technique of forming a SiGe film in a thickness of several xcexcm to increase the resilience of the SiGe film, thereby relaxing the strained SiGe film. For example, Y. J. Mii et al. published the relaxation of a strained SiGe film by forming a SiGe film of about 1 xcexcm thickness with a increasingly graded Ge concentration profile in Appl. Phys. Lett. 59(13), 1611(1991)].
Also, for relaxing the strain of a thin SiGe film, there is known a technique of producing misfit dislocation at a SiGe film/Si substrate interface by high-temperature annealing after implantation of ions such as hydrogen ions and thereby causing the slipping of stacking fault in a defect layer in the Si substrate. For example, D. M. Follstaedt et al. published the relaxation of strain by He ion implantation in Appl. Phys. Lett. 69(14), 2059(1996), and H. Trinkaus et al. published the relaxation of strain by H ion implantation in Appl. Phys. Lett. 76(24), 3552(2000).
As a technique of relaxing the strain of a thin SiGe film without implantation of ions such as hydrogen ions, Japanese Unexamined Patent Publication HEI 10(1998)-256169 proposed a technique of forming a Ge layer of 20 nm thickness on a Si substrate, forming thereon a SiGe cap layer of 1 nm or smaller thickness and annealing at 680xc2x0 C. for 10 minutes, thereby relaxing the Ge layer.
Further, Sugimoto et al. published a technique of relaxing strain in the 31st workshop material, page 29 of the 154th committee xe2x80x9cSemiconductor Interface Control Technologyxe2x80x9d of the Japan Society for the Promotion of Science. According to this technique, a first SiGe film and a first Si cap film are formed on a Si substrate in this order at a temperature as low as 400xc2x0 C., annealed at 600xc2x0 C. to generate a low-density misfit dislocation at a SiGe film/Si substrate interface. Subsequently, a second SiGe film is grown at a temperature as high as 600xc2x0 C. Thereby, an undulation is generated on the surface of the growing SiGe film due to influence of strain fields caused by the misfit dislocation at the SiGe film/Si substrate interface. By compressive stress on troughs of the undulation, dislocation generation sites are newly introduced. Thereby the strain is relaxed while the second SiGe film is grown. According to this technique, threading dislocation in the first SiGe film caused by the misfit dislocation at the first SiGe film/Si substrate interface is reduced by forming the first Si cap film. Further, even if the first SiGe film is formed to have a high Ge concentration (30%), the second SiGe film can be relaxed about 90%.
In the above-mentioned relaxation technique by forming the thick SiGe film to increase the resilience of the SiGe film, an extremely large number of defects are generated in the SiGe film, because the thickness of the SiGe film exceeds a critical thickness for obtaining perfect crystal.
In the technique of high-temperature annealing after implantation of ions such as hydrogen ions, since only the first SiGe film and the first Si cap film form a heterostructure, threading dislocation caused by the misfit dislocation at the SiGe film/Si substrate interface reaches the surface in a high density (about 107/cm2), which results in an increase in a junction leakage current after a semiconductor device is formed. Further, protrusions called crosshatches are produced by the threading dislocation and remaining resilience. In addition to that, if the Ge concentration of the SiGe film becomes high, large holes are liable to emerge owing to hydrogen ions at the SiGe/Si interface, and a very large surface roughness is likely to occur on the surface of the SiGe film.
Further, if the technique of Japanese Unexamined Patent Publication HEI 10 (1998)-256169 is applied to the technique of forming the SiGe film and the Si cap film on the Si substrate to relax the SiGe film, the relaxation ratio declines greatly where the strained SiGe film is thinner than the critical thickness. For example, according to the above 31st workshop material, if the same construction as disclosed by Japanese Unexamined Patent Publication HEI 10 (1998)-256169 is formed under SiGe film formation conditions of a substrate temperature of 400xc2x0 C., a Ge concentration of 30% and a thickness of 100 nm or smaller, which is below the critical thickness, and is annealed at 600xc2x0 C. for five minutes, the strained Si0.7Ge0.3 film is relaxed only about 20%. Therefore, the Si cap film on the top cannot be strained sufficiently, and the carrier mobility cannot be raised to a targeted level.
In the technique of growing the second SiGe film on the first Si cap film/first SiGe film/Si substrate structure while relaxing strain, an undulation of great amplitude (rms: about 9 nm) remains on the surface of the second SiGe film owing to the influence of strained fields by a low-density misfit dislocation and due to film growth at a high temperature.
The present invention has been made in view of the above-discussed problems. An object of the present invention is to provide a semiconductor device and its production process which can achieve a high strain relaxation degree and reduce the threading dislocation density even in a strained SiGe film having a high Ge concentration and a thickness not greater than the critical thickness, and, regarding a second SiGe film formed thereon, can suppress undulation therein, obtain as complete relaxation as possible and improve its smoothness.
The present invention provides a semiconductor device a semiconductor device comprising a first Si1xe2x88x92xcex1Gexcex1 film, a first cap film, a second Si1xe2x88x92xcex2Gexcex2 film (xcex2 less than xcex1xe2x89xa61) and a second cap film formed in this order on a substrate whose surface is, formed of silicon, wherein the first Si1xe2x88x92xcex1Gexcex1 film is relaxed to have substantially the same lattice constant as that of the second Si1xe2x88x92xcex2Gexcex2 film in a horizontal plane.
The present invention also provides a process of producing a semiconductor device which comprises a first Si1xe2x88x92xcex1Gexcex1 film, a first cap film, a second Si1xe2x88x92xcex2Gexcex2 film (xcex2 less than xcex1xe2x89xa61) and a second cap film formed in this order on a substrate whose surface is formed of silicon, the first Si1xe2x88x92xcex1Gexcex1 film being relaxed to have substantially the same lattice constant as that of the second Si1xe2x88x92xcex2Gexcex2 film in a horizontal plane,
the process comprising the steps of:
(a) forming a first Si1xe2x88x92xcex1Gexcex1 film on a substrate whose surface is formed of silicon;
(b) forming a first cap film on the first Si1xe2x88x92xcex1Gexcex1 film;
(c) annealing the resulting substrate to relax the first Si1xe2x88x92xcex1Gexcex1 film so that the lattice constant of the first Si1xe2x88x92xcex1Gexcex1 film becomes substantially the same as the lattice constant of a second Si1xe2x88x92xcex2Gexcex2 film which is to be formed on the cap film and satisfies xcex2 less than xcex1xe2x89xa61;
(d) forming the second Si1xe2x88x92xcex2Gexcex2 film on the first cap film; and
(e) forming a second cap film on the second Si1xe2x88x92xcex2Gexcex2 film.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.